In pendant superheater and reheater units of the convection type, wherein tubes containing saturated vapor at high pressure are heated to impart additional energy thereto, it is known that temperature differentials in the stream of hot gases are a potentially significant cause of premature tube falure. Such failures occur on account of the differential expansion of the tube metal from tube-to-tube within a unit. The temperature differences will cause the tubes to shift out of their original positions producing stresses, and metal fatigue and tube misalignment. The later effect leads to a still further increase in stress as well as erosion and ash build up when the tubes shift out of alignment into the path of hot gases.
In heater designs of the type referred to the tubes are formed in a series of vertically extending straight runs extending parallel to each other. Various spacers of ties have been employed in the past in order to maintain the horizontal spacing between adjacent straight runs. Because of temperature differences from tube to tube, such spacers or ties must allow for differential lengthwise expansion of the adjacent runs. Failure to make allowance for differential vertical expansion by using a spacer or tie which is either welded or otherwise physically joined to the tubes, will set up stresses in the spacer and in the tubular wall material which will eventually lead to failure at the point where one is joined to the other. A failure at this point is likely to result in a rupture of the pressurized tube requiring a shutdown of the entire system.
In an effort to allow for differential vertical expansion of vertically extending heat exchange tubes, spacers such as are shown in U.S. Pat. Nos. 2,477,950, 2,757,649 and 4,412,510 have been provided. These spacers or ties have in common pairs of interengaging elements each of which is welded to one of a pair of adjacent tube walls. Although the elements interengage one another to prevent relative movement in the direction of gas flow, they do allow for lengthwise relative movement of the vapor filled tubes thereby reducing to a degree the cause of failure mentioned above. Despite the fact that these spacers eliminate failure caused by relative differential expansion when the tubes vertically expand in relation to one another, failures persist. A careful analysis will show that this is because there is a temperature differential existing from the inside of a tube, extending through the tube wall and through the physically connected spacer element to the point where it contacts the second spacer element. This differential eventually causes metal fatigue at the point of attachment between the spacer element and the tube wall which weakens the wall and may cause failure of the tube at that point. The condition is exaggerated when the spacer is constructed from a metal different from the tubes. The above referenced U.S. Pat. No. 4,412,510 offers a partial solution to the problem in that adjacent tubes are maintained in tangential relationship thereby minimizing tube to tube temperature differentials. Nevertheless, it has been found that the temperature gradient through the welded tie means and tube wall remains a significant source of failure at the weld.